An exhaust gas of an internal combustion engine contains toxic substances such as NOx. It is known that a NOx catalyst for purifying the exhaust gas of NOx is provided in an exhaust system of the internal combustion engine in order to reduce discharge of these toxic substances. This technology might cause a case in which when a temperature of the NOx catalyst is low, a NOx purifying efficiency decreases. It is therefore required that the temperature of the NOx catalyst be raised up to a temperature enabling the exhaust gas to be sufficiently purified of NOx.
Further, for example, in the case of providing an occlusion-reduction type NOx catalyst, a purifying capacity decreases due to an increase in quantity of the occluded NOx, a reducing agent is supplied to the occlusion-reduction type NOx catalyst, and the NOx occluded in the occlusion-reduction type NOx catalyst is reduction-released (which will hereinafter be termed a [NOx reduction process]). Further, so as to overcome a SOx poisoning which causes a decline of the purifying capacity due to the occlusion of SOx existing in the exhaust gas into the occlusion-reduction type NOx catalyst, in some case reducing agent is supplied to the occlusion-reduction type NOx catalyst and the temperature of the occlusion-reduction type NOx catalyst is raised (which will hereinafter be referred to as a [SOx regeneration process]).
On the other hand, in a particulate filter (which will hereinafter simply be called a [filter]) that traps particulate matters contained in the exhaust gas of the internal combustion engine, if a quantity of accumulation of the trapped particulate matters increases, a backpressure in the exhaust gas rises due to clogging of the filter, and engine performance declines. Accordingly, a temperature of the filter is increased by raising the temperature of the exhaust gas introduced into the filter, the trapped particulate matters are removed by oxidation, thereby scheming to regenerate the exhaust gas purifying performance of the filter (which will hereinafter be termed a [PM regeneration process]).
As described above, the NOx catalyst and the filter each serving as the exhaust gas purifying device are required to be heated in many cases. In this respect, there is an instance of providing a catalyst device bearing an oxidation catalyst and incorporating an electric heater (EHC: Electrically Heated Catalyst) in an exhaust path on an upstream side of the exhaust gas purifying device such as the NOx catalyst and the filter described above. In this instance, when raising the temperature of the exhaust gas purifying device, the oxidation catalyst is activated by electrically heating the catalyst device attached with the electric heater, and the temperature of the exhaust gas purifying device is raised by utilizing the heat of the catalyst device attached with the electric heater. The electric heater, however, entails much consumption of the electric power, and the case is that a load on the battery is increased by frequently using the electric heater. Moreover, in the case of strengthening the battery and the alternator, this is an obstacle against cost-down and downsizing of the apparatus.
Concerning the technology described above, as disclosed in Japanese Patent Application Laid-Open Publication No. 9-158716, such a technology is proposed that time for supplying the electrical power to the electric heater of the catalyst device is determined based on a charging state of the battery. This technology, however, has an instance that the operation of the electric heater is restricted depending on the charging state of the battery, with the result that a temperature of the exhaust gas purifying device is hard to rise sufficiently.
Further, Japanese Patent Application Laid-Open Publication No. 9-217636 discloses a technology, wherein during a period of supplying the electric power to the electric heater for the catalyst device attached with the electric heater, number-of-revolution-of-engine variable means increases the number of revolutions of the engine, and quantity-of-power-generation variable means increases a quantity of power generation by a generator. In this technology, however, the number of revolutions of the engine changes due to the power generation, and hence, if the number of idling revolutions fluctuates especially during an idling operation, an uncomfortable feeling might be given to an vehicle occupant.
Moreover, in a technology disclosed in Patent Application Publication No. 2847976, a voltage of the battery is detected, and a duty ratio of the electric power supplied to the electric heater is acquired from the detected battery voltage and from a predetermined heater voltage. Then, battery energizing time supplied (the time for which to supply the electricity) to the electric heater is duty-controlled at the acquired duty ratio, thereby supplying a fixed amount of electric power to the electric heater.
Moreover, a technology disclosed in Japanese Utility Model Application No. 2548065 is that the internal combustion engine is controlled based on the electric power of the battery, the electric power of the alternator and that of the electric heater for the catalyst.